Magnetic recording and reproducing



MAGNETIC RECORDING AND REPRODUGING Filed Deo. 3,6, 1.940 6Slzleezs-Shee'h 2 ATTORNEY MAGNETIC RECORDING AND BEPRODUCING Filed.Decu 16, 1940 6 Sheems-Sheet AIV/2l A/V/l 'IH 69 .IH @a fe e T wg@eff/mf S MbJbW/Vl@ ATTORNEY MAGNETIC RECORDING AND REPBODUCING FiledDec. l5, .1940 6 Sheets-Sheet llllllllllllllf INVENTOR owp MEQ,

ATTORNEY Aug., 249 M4@ s, J. SEGUN MAGNETIC RECORDING AND REPRODUCINGFiled Decu 16, 194,0 6 SheetsSheei; 5

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MAGNETIC RECORDING AND REPRODUGING Filed. Dec.. 16 1940 6 Sheets-Smet 6Sm BY MMR-1M ATTORNEY Patented Aug. 24, 1943 UNITED STATES PATENT OFFICEMAGNETIC RECORDING AND REPRODUCIN G Semi Joseph Begun, Cleveland, Ohio yApplication December 16, 1940, Serial No. 370,417

17 Claims. (Cl. 179-1003) This invention relates to magnetic recordingand reproducing, and it has among its objects .improved novel magneticrecording and reproducing methods and arrangements which areparticularly useful for applications, such as time delaysystems,although the various features of the invention are not limited thereto.

'lh'e foregoing and other objects of the invention will be bestunderstood from the following description of exempliilcations thereof,reference being had to the accompanying drawings wherein Fig. l. is adiagrammatic view of the circuits and the operating elements of anendless magnetic recording arrangement designed to supply artificialreverberation in accordance with the principles of the invention;

Figs. to 4 are diagrammatic views similar to Fig. l illustrating otherexemplications of the invention;

Fig. 5 is an elevational View illustrating the structural features of anendless tape magnetic recording device designed in accordance with theprinciples of the invention;

Fig. 6 is a top view of Fig. 5 with' parts shown in section;

Fig. 7 is an elevational view of the switching unit associated with therecording device of Fig. 5;

Fig 8 is a side view of the switching unit shown in Fig. '7;

Fig. 9 is an elevational view of another form of endless magneticrecording device exemplifying another form of the invention;

Fig. lo is a side view of the devices shown in Fig. 9;

Fig. ll is a cross-sectional view along line l I i i of Fig. 10;

Fig. 12 is a cross-sectional View along line |2-l2 of Fig. 10;

Fig. i3 is a perspective view illustrating the method of winding theendless magnetic tape on the device of Figs. 9 and 10; and

Fig. 14 is a perspective view illustrating th'e driving arrangement ofthe device shown in Figs. 9 and l0.

Time delay systems have been long known in the communication field.Thus, for instance, Mills Patent 1,647,242 describes how a time delaynetwork consisting of inductances and condensers may be utilized forsupplying artificial reverberation in the reproduction of sound forbroadcasting, sound recording and the like. It has also been longrecognized that if a. longer time delay, for instance, of the order of1,/4 of a second or more is required, time delay systems, formed ofelectric network elements only, are very expensive and bulkyparticularly if used in the lower frequency range, such as ataudiofrequencies, requiring magnetic core inductances.

It has also been suggested that under conditions requiring such longertime delays, sound recording sh'ould be used instead of electric networkelements as the time delay medium. Thus, for instance, Arnold Patent1,859,423 proposes the production of reverberation effects by recordingthe sound and reproducing the record with a plurality of pickups locatedat closely succeeding intervals along the sound record track andcontrellably attenuating and combining the outputs oi the pickups so asto reproduce the recorded sound with delayed echo and reverberationeffects.

Although disc recording and other long known sound recording media, suchas magnetic recording, which enable instantaneous playback of recordedsound, make it possible to add articial reverberation eiects to soundsequences, prior efforts to use sound recording for applications, suchas artificial reverberation systems, never came into use because thediscontinuity between the end and beginning of a sound track formed onsuch recording medium make it impossible to use such time delay systemsfor supplying the reverberation effects during prolonged continuousperiods.

Although an endless magnetic recording tape might have seemed to beuseful as a continuously operating sound recording medium suitable forartificial reverberation systems, the discontinuity at the junction orthe soldering point which has to be provided in order to join the endsof a length of magnetic tape into an endless tape would, on passingthrough the large number of closely spaced playbacks required to supplythe prolonged artificial reverberation time delay effects, induce ineach playback head disturbing high electric potentials, therebyintroducing into the playback reverberation channel a series ofdisturbing clicks. -As a result, such endless magto devise an articialreverberation system utilizing recording on an endless phosphorescentrecord surface as the time delay medium. However, althoughphosphorescent sound recording media have the advantage that they may bereadily designed to form a continuous endless sound track without anydiscontinuity, the practical adaptation of such phosphorescent surfacerecord media for artificial reverberation encountered great difilcultiesand have never reached practical acceptance.

One phase of the invention is based on the discovery that an endlessmagnetic tape with a soldering Junction discontinuity may be used forsupplying artificial reverberation during prolonged continuous periods,and that such endless tape magnetic recording medium may be designed soas to suppress the disturbing effect of the soldering junction withoutin any way diminishing the effectiveness of the endless tape recordingmedium in producing artificial reverberation eifects.

If sound proceeds from a source located in a room and strikes a wallsurface, it is partly reflected therefrom and travels until it reachesanother surface where it is again partly reflected.

the recording head may be used, or the output of the amplifier whichdrives the recording head is combined with the blended output of theseries of playback heads which reproduces the recorded sound with anexponential attenuation eiiect.

In the simplest form of an endless magnetic tape recording machinedesigned to supply reverberation in accordance with the invention, anendless magnetic recording tape is continuously moved during each cycleof its total recording length through an obliterating head, a recordinghead, and then in sequence through a series of closely spaced playbackheads so that each successive playback head reproduces the recordedsignal with a greater time delay, and the output of each playback headis exponentially attenuated with relation to the preceding playbackhead, so that the combined output of all the At each sound impact, someof the energy of the sound is absorbed by the wall surface and thereflected sound is weaker than the original. Such action results in asuccession of tones of diminishing intensity which give the listener theimpression that the original sound dies out slowly, and is known asreverberation. The reverberation time of a room is usually defined asthe time necessary for the sound of a source, that is suddenly stopped,to decay 60 decibels.

In many applications, it is desired to change the reverberation time ofa room. Orchestra music, for instance, has more appeal if it is richerin reverberation, and the voice of a singer is more pleasing if it isheard with less reverberation.

The absorption of the sound in a room will depend on the vapor contentof the air, particularly for high frequencies, and on the absorptioncharacteristics of the walls, and the fumiture in the room. Accordingly,the sound decay will vary for different frequencies. In general, therate at which the sounds of the diil'erent frequencies decay has anexponential characteristic, and the higher frequencies have a fasterdecay time than the lower frequencies.

A satisfactory artificial reverberation device should, therefore,operate in relation to a constant frequency impulse as follows: At thetermination of the sound impulse, the device should bring about agradual decay of the impulse from full amplitude to inaudibility.

The magnetic recording device of the invention is designed to functionas a time-delay medium operating in accordance with the followingprinciples: A recording head and a plurality of magnetic playback headsare placed successively along portions of an endless magnetic tape, sothat the sucessive playback heads reproduce each recorded sound impulsewith a greater time delay, and the output of the successive heads isexponentially attenuated so that the combined output of all the playbackheads shall blend and, on being converted into sound. create theillusion of an exponentially decaying true reverberation effect.

For reproducing sound with reverberation, the output of the amplifierwhich output of an additional playback head placed immediately behindsible the exponential decay of natural reverbera-A tion However,assuming that such conditions would be possible, and that the outputs ofthe playback heads are not attenuated, their mixed output will alwayscontain two output components which are reproducing voltages oi' thesame amplitude, out of phase, thereby cancelling the signal. Thisreasoning applies for any frequency. An arrangement of this character,even if the output of the successive playback heads is subjected tosuccessively increased attenuation, would actually operate as afrequency discriminating system which attenuates high frequencies morethan low frequencies. Accordingly, such system would be undesirable.

With respect to the spacing between the playback heads, the yfollowingconsiderations are of great importance: If a record of a short suddensound impulse is played back with time delay intervals betweensuccessive playback heads, a listener will distinguish individualrepetitions of the short signal rather than the blending of thesuccessive playback outputs if the time delay interval between twoadjacent heads is too large. I have found that in order to blend thetime dc= layed outputs of the successive pickup heads in a manner thatwould create the illusion of natural reverberation, it is important thatat least the first few playback heads should be spaced as closely aspossible following the recording head, and that the distance betweenthem, measured in the time delay intervals, should be of the orderbetween about 1/zo to 1/m of a second or even closer, if possible.

I have found that by taking advantage of the fact that even a smallstudio has a certain small reverberation time, a universally adaptableendless magnetic recording tape reverberation device may be provided ifthe playback heads are placed sufficiently close to play back therecorded signal with successive time delays of the order of betweenabout 1,oth and 1,40th of a second between the first group of successiveplayback heads, which contribute the major part of the reverberationoutput with an attenuation up to about l2 decibels relatively to theoriginal signal level, although relatively good results are obtween theplayback heads which deliver the recorded signal output with anattenuation up to about 6 to 8 decibels.

Satisfactory operating conditions have been obtained by spacing the rstten playback heads so that each picks up the recorded signal with a timedelay of approximately /Qth of a second relatively to the precedingmagnetic head, and. that the additional playback heads picks up therecorded sound with successive time delays ci -sth of a second, and thatthe last playback head picks up the signal about one second after it wasrecorded.

By increasing the attenuation of the output of the nrst ten playbackheads in 11/2 decibel steps, that is, attenuating the output of the rsthead with relation to the original signal lll/ decibels, and the secondhead 3 decibels, and each successive head upto the tenth head .l1/2decibels per step, the output level of the tenth head is l5 decibelslower than the original signal level. The output of the next iour or veplayback heads may be attenuated 3 decibels per step so that the lasthead is about 30 decibels attenuated below the original signal level.

The outputs of the diierent heads are then combined in a mixer amplifierand the output in this amplifier again is combined in properrelationship either with the original signal or with the signal suppliedby a playback head placed in ahead ci the `hrst reverberation playbackhead.

In designing such a system, the following precautions must be observed:The signals contributed from each cf the pickup heads must be mixed sothat one does not interfere with the signal obtained from anotherplayback head. 'io this end, the output of each playback head may beimpressed on two series connected resistance elements, one being a fixeddecoupling resistor, the other being a variable tap resistor, so as topermit proper adjusting of the'attenuation while making it impossiblefor a signal picked up by one playback head from being impressed onanother head and cause it to record the impressed signal.

ln the arrangement shown, satisfactory operating conditions wereobtained by connecting the first nve pairs ci' magnetic heads in seriesacross a potentiometer element formed of a xed dew coupling resistanceand an adjustable tap resistor. The variable tap resistance section ofeach potentiometer unit is designed so that in the successive stages itgives at its terminals a smaller output voltage in the ratio describedabove, the overall resistance of each potentiometer remaining the same.l

Since in an endless magnetic tape time delay recording device requiredfor producing rever-y beration eiects, the soldering junction must passthrough a large number of magnetic heads, it causes not only electricaldisturbances, but the mechanical impact of the soldering junction on thethin pole pieces of the magnetic heads imposes thereon also seriousmechanical strains which aect their magnetic properties.

I have discovered that the disturbing effect of the soldering point maybe completely eliminated without in any way diminishing the illusion ofthe artificial reverberation produced by an endless magnetic tape, ifthe output of the successive playback heads is cut ofi during the periodwhile the soldering point passes through them. particularly if the timerequired for the soldering point to pass through the succession ofplayback heads is less than about 50 to 100 times the total recordingperiod of the endless tape.

I have found that for best performance, the endless magnetic tape used'in such recording arrangement should have a recording length ot' atleast of the order of 1/2 of a minute, but preferably at least of theorder of one minute.

l have also found that itis not necessary to cut off the output of eachplayback head individually at the moment when the soldering joint passesthrough it, and that very good results are obtained ii the output of alladjacently placed playback heads used for producing the reverberationeffect are cutoi momentarily for a short instant when the solderingjoint reaches the pole piece of the iii-st playback head and provisionis made to gradually bring up the level of the output as the solderingjoint moves from the heads which operate with the smallest attenuationand are closest to the recording head toward the farther removedplayback heads which operate with the maximum attenuation, since themuch greater level of the first group of playback heads is notperceptibly affected by their output while thepiier is cutoff completelyonly While the solder ing joint is moving through the rst few playbackheads which contribute the major portion ofi/the output level, and afterthe soldering joint has passed through them and moves through theplayback heads which contribute only the minor fraction of the outputlevel, the gain of the am pliiier is gradually restored, restoring theeffectiveness of the nrst few magnetic heads which contribute the majorpart of the output level, and the restored gain is not suilicient topermit the soldering joint to cause any disturbing effects while it ispassing through the last few heads, their amplined disturbing signalbeing kept sufficiently down below the level of the signal contributedby the first playback heads.

According to another phase of the invention, the eect of cutting out theoutput of the playback heads on the overall reverberation eiect isfurther reduced by distributing the magneticv playback heads requiredfor producing the desired reverberation effect on a plurality ofdifferent sections of the endless magnetic tape, so as to provide eachtape section with its subgroup of playback heads and its own recordinghead. which are cooperating to supply complementary parts of the overallreverberation effect.

Thus, if an endless magnetic tape, having 20 magnetic playback headsreproducing the recorded signal with time delays of 1/20th of a second,is to be used for supplying articiai reverberation, the magnetic headsare distributed not on one continuous section of the endless tape, but,for instance, on four sections as follows: On the first section, arecording head with playback heads reproducing the signal with l, 5, 9,13 and 17 times greater delay than the minimum time delay of l/20th of asecond; on the second section, a recording head with playback headsreproducing the signal with 2, 6, l0, 14 and 18 times greater delay; onthe third section, a recording head with playback heads reproducing thesignal with 3, 7, il, l5 and 19 times greater delay; and on the fourthsection, a recording head with playback heads reproducing the signalwith a, 8, l2, 16 and 20 times greater delay.

have found that an endless magnetic tape reverberation equipment Willsupply an illusion of a 60 decibel reverberation time, although onlyenough playback heads are used for playing back the recorded signal witha gradual attenuation down to 30 decibels only, dispensing withadditional playback heads that would be required in order to attenuatethe original signal to full 60 decibels.

An endless tape magnetic recording time delay machine designed forsupplying artificial reverberation may also be used for producingartificial echos or other sound effects. Thus, by providing an endlessmagnetic tape recording machine designed for supplying articialreverberation with one or more recording heads located a distance aheadof the reverberation recording head used for producing artiiicialreverberation by a distance corresponding to the desired echo time, suchmachine will operate as an echo machine if the additionally providedrecording heads are used in lieu of the reverberation recording head.For instance, if an echo effect of one second is desired, the additionalrecording head is displaced relatively to the iirst of the succession ofplayback heads used for producing artificial reverberation by a distancecorresponding to one second.

When using such endless magnetic tape recording device for producingecho effects, the limitations with regard to the exponential attenuationoi the successive pickup heads do not apply, and the ratio of the outputlevel of the playback heads relatively to the recorded signal may be sochosen as to give the desired echo or other sound effects.

The output channel of the playback heads used for producingreverberation, echo and other sound eiiects may be provided withsuitable frequency discriminating networks so as to make lt possible tomodify the frequency characteristics of the output in any well knownWay.

An endless magnetic tape reverberation equipment of the inventionpermits ready change of the reverberation time and may be operated tosupply a short reverberation time, for instance, if the voice of asinger is to be transmitted, and a longer reverberation time, forinstance, if the music of an orchestra accompanying the singer is to betransmitted. In such case, one microphone setup may be used for pickingup the voice of the singer, and another microphone setup may be used forpicking up the music of the orchestra, and two different reverberationeffects, one with a shorter reverberation time and one with a longerreverberation time, are combined with the outputs of the two microphonesetups.

A single endless magnetic tape driven by a single motor drive system maybe used for operating one or more of such reverberation systemsoperating with diierent reverberation times.

A11 endless magnetic tape recording equipment exemplifying one practicalform of a time delay arrangement which in actual operation proved to bevery effective in supplying a perfect illusion of actual reverberation,is illustrated diagrammatically in Fig. l. In the form shown, amicrophone MC, representing a setup of several microphones, impressesits electric output, generated by the sound to which it is exposed, intoelectric energy impressed through input lead 2l on the input side of apre-amplifier IA. 'I'he pre-amplier IA-impresses the amplied microphoneoutput through a direct channel lead 22 by way of an intermediateconstant impedance attenuator 23 and an attenuator decoupling networkATD-l to an output lead 24 which is connected, for instance, to amodulator transmitter equipment MO'IR of a broadcasting station, so asto transmit a radio broadcast modulated by the sound to which themicrophone MC is exposed.

In addition, the pre-amplifier IA also impresses the ampliiiedmicrophone output through a reverberation channel lead 25 on a timedelay or artiiical reverberation equipment RE, the output of which isimpressed on amplifier PMA, the output of which is impressed througha'reverberation channel output lead 26 and a constant impedanceamplifier 21 connected in series with an attenuator and decouplingnetwork ATD-2 on the common output lead 24, so that the radio broadcasttransmitter shall be modulated by the combined output of the directtransmission channel and the reverberation transmission channel.

In the form shown, the time delay or reverberation equipment REcomprises an endless magnetic tape 3| driven by a guide roller 33engaging a portion of the tape and driven by a suitably energizedelectric motor 34, for instance, so as to impel the tape 3| at; auniform speed, for instance, in clockwise direction indicated by anarrow. The endless magnetic tape recording arrangement is preferably ofthe type disclosed in my copending application Serial No. 197,851, tiledMarch 24, 1938 as a continuation-in-part of my application Serial No.82,962, tiled June 1, 1936, which will issue as Patent 2,224,854, or itmay be of the type disclosed in my copending application Serial No.359,177, led September 23, 1940.

An endless magnetic recording medium, such as the tape Si, has to beformed by joining the ends of a long tape of magnetic steel, throughsoldering, for instance, at a soldering junction 35, although thevarious features of the invention are not limited to an endless magnetictape of this particular material. The endless tape is guided and drivenat a uniform speed and, in the form of arrangement shown, three sets ofobliterating and recording heads OA, OB1 OC, RA, RB, RC, respectively,are placed along diierent sections of the endless tape so that uponpassing through the magnetic heads OA, RA, a new signal will be recordedthereon by the recording head RA, and after an interval, upon passingthrough the obliterating and recording heads, OB, RB, another signalwill be recorded thereon by the recording head RB, and after a furtherinterval, another signal will be similarly recorded thereon by themagnetic head RC.

As explained hereinabove, to produce the desired articial reverberationeffect, a set or array of playback heads has to be placed following oneof the recording heads, such as RA, so that the recorded signal shall bereproduced by the successive playback heads with successively greatertime delays. For instance, assuming that a series of successive timeintervals are indicated in Fig. 1 by the distances between a series oftime interval double-dot dash lines T- to T-25, extending transverselyto the indicated directions of the motion of the tape, and a set ofplayback heads Pl to Pl4, such as indicated on the drawings is placedsuccessively in the direction of the motion of the tape immediatelyfollowing the recording head RA, the successive playback heads willreproduce the recorded signal with a successively greater time delayrepresented by the distance of the successive time interval linesI T-lto T-25 from the zero line T-ll.

According to one phase of the invention, the same effect is obtained bydistributing the set of time-delay playback heads Pl to P|4 into anumber of subgroups or subarrays, for instance, as shown, in threesubgroups, and arranging them so that, with the three recording headsRA. RB, RC recording simultaneously the same signal, the signal recordedby the recording head RA is re produced only by the playback heads PI,P2, Pl, P8, Pl2 with time delays indicated by the time interval linesTl, T-2, T-fl, T8, T-I'l;

that the signal recorded by the recording head RB is reproduced onlywith the playback heads P3, P4, P9, Pill, Pl3 so that they play backwith time delays indicates by the time interval lines T-3, JT-4, 'lL-9,IL-l0, 'Ilm-2i; and that the signal recorded by the recording head RC isren produced only by the playback heads P5, P6, PI i, PM with timede'iays indicated by the time interval lines TM5, T-S, IL-E3, 'IL-25,the combined output of all playback heads Pi to Pit being equivalent tothat obtained if all playback heads are placed so that they reproducethe signal, for instance, recorded by the head RA, with the indicatedsuccessively greater time delays IL-i to 'Tm-25. Taking the timeinterval represented by the distance between lines T-il and 'i1-i as/zoth oi a second, the array of the time intervals Tim@ to T-brepresents the time intervals oi a practical reverberation eduipmentwhich proved successful in actual operation.

Such distribution of the playback heads along diiferent endless magnetictape sections for cooperation with different recording heads so as toreproduce the same signal with successively greater time delays, or, ingeneral, to give the same overall effect as if all the playback headswere arranged to reproduce the signal of a single recording head, hasmany advantages.

Thus, the frictional forces exerted by the pole pieces of the playbackheads are distributed on a greater length of tape and thereby reducesthe strain imposed on the tape, and keeping below the magnitude thatwould impair its magnetic as Well as mechanical properties.

This arrangement makes it also possible to impart independent drivingforces to each tape section passing through the individual magneticheads, for instance, by applying the force of a driving roller to eachof the sections of the tape passing through the diierent subgroups ofplayback heads, thereby reducing the total driving tension which has tobe imposed upon the tape.

Furthermore, the greater spacing with which the successive playbackheads of each subgroup have .to operate enables also closer spacing ofthe playback heads of each subgroup, or a larger number of playbackheads for producing the same overall time delay eiect than could bepossible otherwise, and thus makes it possible to ren duce vthe timedelays between the outputs of the successive heads of the sequence andgive better blending of their outputs. With the greater spacing betweenthe successive playback heads thus made possible, the problem ofmechanical mounting of the playback heads and their crowding, iseliminated.

The foregoing advantages apply to the distributions of the severalsubgroups of playbacks, either on dierent sections of a single endlessmagnetic tape, or on several distinct endless magnetic recording tapesmoving at the. desired speed, or different speeds. In addition, suchvdistribution of the magnetic playback heads makes it possible todecrease the disturbing effect of the soldering joint discontinuity onthe reverberation supplied by the equipment. Each of the array oftime-delay playback heads forming part of such reverberation equipment,is associated with a suitable attenuator so that the output componentderived from each playback head has the proper exponential in decayrelation with respect to the output component supplied by the precedingplayback head, suitable decoupling means, such as decoupling resistors,being combined with such attenuator so as to prevent the output of oneplayback head from becoming impressed on another playback head andbringing about a recording of the impressed signal.

In the practical form of reverberation equipment illustrated in thedrawings, it was found sumcient to provide a single attenuator unit forthe playback heads which operate with a smaller time delay. Thus, asshown, the pairs of playback heads Pi--P2, 'P3-P4, P5-P, P'IPB, Pil-Pit,which operate with shorter time delay intervals, are connected in seriesto the attenuator units Al, A34, A56, A18, ASU, respectively, while theplayback heads Pl i to PM. which operate with larger time delayintervals, are each connected to a separate attenuator unit Ai, All, A2i, A25, respectively.

Each attenuator unit is provided with an ad-z justable rheostat element3? connected in series with a decoupling resistor 38, which are soproportioned that the fraction or' the output voltage of the playbackheads applied to the rheostat elements 3l of the successive playbackheads A42 is attenuated to a greater degree with generally exponentialcharacteristics in the way indicated by the increased length ofdecoupling resistor` elements 38 of each attenuator unit.

Various ways may be used for combining the outputs of the successivetime delay playback heads, for instance, by decoupling networksinteconnecting the various heads. If all the playback heads reproducethe signal recorded by a single recording head, the attenuators of allthe playback heads may be interconnected so as to add fractions of theiroutputs in series and combine them in the way indicated in the drawings,the so-combined or mixed outputs of all the playback heads cooperatingwith the same recording head being applied to an input stage of theplayback amplifier PMA.

In an arrangement, such as shown in Fig. l, in which the array ofplayback heads required for producing the overall reverberation effectis segu regated into a plurality of groups, cooperating with differenttape sections, each associated with a separate recording head, thecombined attenuated output of the playback heads associated with theindividual diierent recording heads may ndividually be impressed on adierent input amplier stage 40A, lll-B, dll-C, so as to form separatechannels in the way shown. The outputs of the three input amplifierstages dll-A, dll-B, 40C of the three channels, are suitably mixed andfurther amplied, for instance, by the playback and mixing amplifier PMA.

In combining the outputs of the three tubes 4I, it is important to makeit impossible for the out put of one channel to modify the operation ofthe other channel. Any arrangement which combines the output of thethree channels with suitable decoupling means may be used for thispurpose. In the arrangement shown, the output circuits of the threetubes are coupled through independent transformers, the Seoondaries ofwhich are connected in series and are feeding into the mixing amplifierPMA. In order to eliminate any interference between the microphoneampliiler IA on one hand and the pickup mixing amplifier on the otherhand, decoupling attenuator networks are provided in the direct channeland in the reverberation channel. The pickup mixing amplifier PMA isequipped with a frequency discriminating network or tone control so asto give the reverberated signal any desired frequency response, thusbeing able to simulate sound effects in a reverberated hall having wallsof different sound absorbing and sound reflecting characteristics, suchas a large church with a dome, or a large auditorium. i

As shown in Fig. l, the input stage of such ampliiler may comprise, forinstance, an amplifier tube 4I having a cathode 42, an anode 4I and acontrol grid '44, the cathode being shown, for instance, connectedthrougha self-biasing resistor to a ground lead G, and the control gridM being connected by way of a conventionalA coupling condenser to theinput lead 48 which impresses thereon the combined properly attenuatedoutputs of the several time-delay playback heads.

Since in the arrangement shown in Fig. 1, the array oi' playback headsPI to PII is divided into subgroups, each subgroup cooperating with aseparate recording head RA, RB,'RC, respectively, three similar inputamplifier stages are provided, and the control grid 4l of each tube issimilarly connected to the serially connected output taps of theattenuator units associated with the respective groups of playbackheads, the low potential side of the rheostat element 3l of theattenuator unit Al, A, AIB associated with the playback heads of eachsubgroup supplying the greatest output component being shown grounded tocomplete the input circuit for each input amplifier stage.

As explained hereinabove, the discontinuity, such as the solderingJunction Il, which has to be provided in each endless magnetic tape,produees a succession of high voltage impulses as it passes through therelatively large number of successive magnetic heads required to supplyarticial reverberation, thereby impressing on the playback channel aprolonged disturbance that has heretofore made it impossible to use anendless magnetic recording tape as a time delay medium for artificialreverberation or the like.

By using a long endless magnetic tape so that, during each completeendless tape cycle, a long interval will pass before the soldering pointpasses through the magnetic head, its disturbing effect is rendered lessobjectionable.' In addition, according to the invention, the disturbingeffect impressed by the soldering junction of an endless magneticrecording tape on the playback channel is suppressed by providingautomatic means for momentarily cutting oil the outputs of the playbackheads without disturbing the overall operating results desired from theequipment.

In the arrangement shown, at a point of the endless magnetic tape 3|which passes the playback heads somewhat later than the soldering joint,is provided a depression 39 cut, for instance, along the edge of thetape, and a switch 5I placed along the path of the motion of the tape isarranged to be actuated by the depression to a position in which itmomentarily cuts off the output of the playback heads and prevents theelectric disturbance induced in the playback heads by the solderingjoint 35 from being impressed on the playback amplifier.

Various arrangements may be utilized for carrying out such controloperation in response to the operation of such cutoff unit. In thearrangement shown, a point between two series grid resistors l2, I3 isconnected through a resistor M to one contact of the switch Il the othercontact of which is connected to a source of negative blocking voltage,indicated by a minus terminal of the playback ampliner PMA, so that whenthe cutoff switch II, which is normally in the open position, ismomentarily released by the tape depression Il to its closed position,it instantaneously applies a negative blocking potential to the controlgrid 44 of the amplifier stage and instantaneously cuts off theamplifying action of the amplifier tube 4 I.

The network through which the negative blocking potential is applied tothe control grid M includes also a condenser 5l, and the elements of thenetwork so formed are so proportioned that when switch 5I, aftermomentarily closing and applying the blocking potential and cutting offthe gain of the amplifier stage I I, is immediately reopened, theblocking potential is not immediately removed from the control grid M ofthe amplifier tube I I, but is only gradually decreased as the .chargeon the condenser I5 leaks .through the resistors 52, M, so as togradually restore the effectiveness of the input amplifier stage, whilethe soldering point 35, having already moved past the flrst group ofplayback heads, is still passing through the remaining playback heads.

However, since the playback heads which are farther away from therecording head contribute only a minor fraction of the output levelcompared to the contribution of the first few mag- -netic heads throughwhich the soldering point has moved while the amplifier stage wascompletely cutoif, the gradual increase in the gain of the amplifierstage-will not be sufficient to cause any disturbing effects while thesoldering point is passing through the last few heads, because theamplified disturbing signal outputs of 4these last few heads hasnegligible effect compared to the amplified signal level derived fromthe first few heads, which operate with the least amount of attenuation.

In other words, while the soldering joint passes the nrst fewreproducing heads which contribute most of the channel output requiredfor creating the artificial reverberation effect, the amplifier iscutoff, and as soon as the soldering joint reaches the heads whichcontribute only a smaller portion of the channel output, the normal gainof the amplifier is gradually restored.

In the arrangement described above, a depression along the edge of thetape located back of the soldering joint is arranged to operate acontrol element located along the edge of the tape so that thedepression actuates the control element to operate for a split secondand by so doing apply a blocking potential to an amplifier whichinstantaneously stops amplification and reduces the gain to zero, incombination with a time delay means, which becomes effective immediatelyafter gain was cutoff to gradually restore the gain at a rate whichassures that the synthetic reverberation effect is cutoff only for anegligibly short time and is restored so quickly that the momentaryabsence of the reverberation effect is not noticeable at all.

In the arrangement shown, the individual rheostat 31 of the attenuatorunits AI! to A25 associated with the individual playback heads may beprovided either with a slider tap or with individual switch taps so thatthey may be controlled all as a unit, and that the attenuation of allthe attenuator umts associated with the playbacks may be simultaneouslyincreased or decreased by operating a single actuating element, such asa handle or push button.

All the rheostat taps of all the playback heads are arranged to becontrolled in common so as to permit simultaneous varying of theattenuation tension of the outputs of the different playback heads, butnevertheless keep their different attenuation levels in the properexponential relation. By moving them all at the same time in onedirection or in the other direction, the reverberation time may thus beincreasedor decreased. Instead of operating them by a continuousmovement, they may be operated by push buttons, marked, for instance,1,41 second, 1/2 second, 3/5 second, l second and 2 seconds. v

Alternatively, suiilcient illusions of a different reverberation timecan be obtained without actually changing the attenuation of the outputsof the individual playback heads, but by adjusting v the reverberationequipment as a whole to give a long reverberation time and changing thelevel ratio of the direct channel output through lead 22, with relationto the reverberated channel output through lead 26. While this procedureis not in accordance with more strict theoretical con siderations, ithas been found that the illusion is generally satisfactory forbroadcasting applications.

In the arrangement'of Fig. l, such attenuation mixers are shown at 23and 2l by combining' in each a constant impedance attenuator connectedin the direct channel with a constant impedance attenuator inthe'reverberation channel, and operating them manually so that While theattenuation is increased in one channel, it is decreased in the otherchannel. v

The endless magnetic tape recording arrangement of the type shown inFig. l may be also utilized for providing a plurality of independentreverberation channels, for instance, one reverberation channel forsupplying one type of reverberation to a singer, another type ofreverberation to the orchestra music and another type of reverberationto a choir. It is merely necessary to provide each tape section, such asshown in Fig. l, with a full complement of magnetic playback heads, sothat each of the three tape sections having its own recording head RA,RB, RC, respectively, operates with a full array of playback heads Pi,P2, P3 Pl3, PH, as an independent complete reverberation channel, thecombined output of all the playback heads of each independent channelbeing impressed upon the initial amplification stage, such as shown atlit-A, for instance, of its own individual playback amplifier equippedwith its tone control and other accessories. The outputs of theamplifiers of each playback channel so formed are then properly combinedthrough a suitable mixer and impressed in the desired proportions on theoutput lead 24, as described above.

The acoustic characteristic of a room are determined by itsreverberation time. This reverberation time depends upon the acousticaltreatment of the walls, iloors, ceiling, furniture, and other object inthe room. If more sound absorbing material is used on the walls,ceiling, etc. of the room, its reverberation time will be shorter, andthe acoustic liveliness of the room is considerably reduced.

In many cases, it is desirable to have flexible room acoustics,particularly if performances of different character take place in thesame room, for instance, public meetings, concerts, theatricalperformances, since music requires different reverberation time thanintelligible speech. In order to modify the reverberation time of aroom, some of the radio broadcasting studios have to be built withmovable walls and special curtains having high sound absorbingqualities. To change the acoustic liveliness of such studio, thephysical dimensions of the room are changed, and the curtains are raisedor dropped. This is not only complicated, but requires time. In addition, such studios are expensive to build and their layout iscomplicated, and their adaptability to diierent conditions is limited.

According to the invention, an endless magnetic tape time delayrecording arrangement of the type shown in Fig. 1 is utilized forvarying the acoustics of a room, such as radio broadcasting studios,without in any way changing its physical dimensions or resorting tofurniture treatment, curtains or the like. The invention is based on thefollowing considerations: The

acoustic liveliness of a room depends on the sound reected from thewalls, ceiling, floor and other objects in the room. and other objectsof the room absorb of the sound energy, the room is completely sounddead.

In accordance with the principles of the invention, a room which issubstantially sound dead is provided with different sources of sound,such as loudspeakers, distributed at diiierent points of the room andoperated by a time delay sound recording arrangement so as to reproducesound from a source, such as microphones suitably located in the room,an effect that would be produced in such a room if the room would be ofdifferent dimensions or `have different Wall treatment. Sound from asound source placed in a room equipped with such artificial acousticsystem will travel in all directions until it reaches the boundaries ofthe room where it will be completely absorbed, but at the same momentthe sound delay recording system to which the loudspeakers, distributedalong the walls of the room, are connected, will reproduce sound of thedesired reilected character, such as would be produced by the source ina room of different dimensions having a special acoustic treatment ofits interior.

The loudspeakers for producing the illusion of an acoustically aliveroom are distributed around the walls and the ceiling, and at least oneloudspeaker is placed in the neighborhood of the source of sound where,for instance, the microphone setup is placed. The microphone ormicrophone setup is connected through suitable ampliers to a number oftime delay sound recording channels, and these sound delay channels areconnected to the individual loudspeakers, the individual channels beingprovided with frequency discriminating and attenuating networks so thatthe individual loudspeakers reproduce the sound so that the overallsound delivered from the loudspeakers has the same character ofreverberated acoustic energy.

In other words, the reverberation channels connected to the differentloudspeakers are so designed and arranged as to deliver through theloudspeakers sound which, when blended, will create in the room theillusion of the selected desired character of liveliness. Thus, a roomprovided with an artificial acoustic treatment system of the inventionmay be given acoustic characteristics of a -big church or a very smallintimate room. It is merely necessary to know If the walls, ceiling thereverberation time of the particular church and the particular smallroom, and adjust the electrical control elements of the reverberationchannels to operate with the same reverberation time, and supply theloudspeakers with an output having the same frequency characteristics.

Fig. 2 illustrates diagrammatically and in simplified form oneexemplication of an artificial room acoustic system. A room 60, havingsound dead interior walls, is provided with a loudspeaker RO locatednear the source of sound, for instance, a speaker, next to whom amicrophone MC is placed, and several groups of loudf' speakers Ri, R2,R3, etc. are distributed along the walls of the room.

In the arrangement shown, it is assumed that the microphone MC islocated at the source of sound. The loudspeakers R2 are shown locatedalong points of the wall which will be reached by sound from the sourcein two time intervals required for the sound to reach the loudspeakersRi, and three such time intervals will be required for the sound toreach the location of the loudspeakers R3. There is also provided amagnetic recording time delay arrangement comprising an endless magnetictape 32, such as shown in Fig. 1, driven at a uniform speed in clockwisedirection, for instance, past an obliterating head OA, a recording headRA, and a succession of playback heads PI to P14, for instance, so thatthe playback heads reproduce with a series of time delay intervals thesound recorded by the recording head, which is shown connected to anoutput lead from an amplifier IA, the input lead 2i of which isconnected to the microphone MC.

The magnetic recording arrangement operates similar to that described inFig. 1, and is provided with a cutoff switch 5l for momentarily cuttingofi? the output of the playback heads at the moment the solderingjunction reaches the first playback head. Since the several sets ofloudspeakers Ri, R2, R3 are shown symmetrically located in the room, thereverberation effect of the room will be symmetrical, and it issuiilcient to operate each of the groups of loudspeakers from the sameenergy source.

The loudspeaker R0 which is located adjacent the sound source, should besupplied directly with unmodified amplified energy representing, ineffect', the usual public address system loudspeaker. The three groupsof loudspeakers RI, R2, R3, are supplied through three dierentreverberation channels which may be designed to have the same'reverberation time, but they should distinguish from each other asfollows: The group of loudspeakers Rl which are closest to the soundsource, should reproduce the sound with a time delay corresponding tothe time interval required for the sound of the sound source to reachthem. The loudspeakers R2, which are spaced from the source by a timeinterval that is twice greater, should reproduce the sound with a twicegreater time delay interval, and the loudspeakers R3 with a three timesgreater time delay interval. Assuming, for instance, that the sound hasto travel from the sound source to the speakers Rl l/nth of a second, tothe speakers R2 fnth of a second and to the speakers R3 1,4th of asecond, the speakers RI will bevsupplied through different reverberationchannels corresponding to these requirements.

According to the invention, the output of the individual playback headsof an endless magnetic tape time delay recording device designed forsupplying artiilcial reverberation, for instance, is utilized forsupplying diierent reverberation channels with time delayed playbacks ofthe recorded sound, each channel being independently controlled tooperate with different reverberation characteristics.

In order to produce the eil'ect of the three diiierent reverberationchannels, it is merely necessary to combine fractions of the output ofthe successive playback heads PI to PM which reproduce the signalrecorded by the recording head so as to operate in the same way as ifthree separate sets oi playback heads were used for providing threedifferent reverberation channels, in the way described in connectionwith Fig. l, for instance. Thus, assuming that the time delay intervalbetween the successive playback heads PI to PH is 1,-/20th of a second,then for supplying the reverberation eifect required for theloudspeakers Rl, a component of the output of all the playback headsshould be combined; for supplying the reverberation eiiect required forthe loudspeakers R2, a portion of all the playback heads, except theiirst one, should be combined; and for supplying the reverberationeffect required for the loudspeakers R3, a portion of the output of allthe playback heads, except the ilrst three playback heads, should becombined.

Various known ways may be used for supplying a portion of the output ofthe same playback head to diierent ampliiier or reverberation channels,while assuring that no energy is fed back to the playback head. In thearrangement shown, the outputs of the various playback heads arecombined without using special decoupling networks by providing the polepiece of each head With a plurality of distinct windings and connectingthem into different channels with the properly gradated attenuation toproduce the desired reverberation effect,

In the arrangement shown in Fig. 2, the three sets of loudspeakers RI,R2, R3 are connected to three output leads El, 62, 63 of the threeplayback amplifiers PB-L PB-2, PB-3, on

the inside of which is impressed the input oi three differentreverberation channel leads 46-A, 46-B, 46C, respectively As shown inFig. 2, each playback head Pl to PH has a pole piece interlinked withthree output coils ll, l2, 13, each connected to an attenuator unitformed of an adjustable rheostat element 31 and a decoupling resistor 38designed so that the output applied to the resistance element of .thesuccessive heads is exponentially attenuated relatively to the precedingplayback head.

In order to supply the reverberation channel lead A with the requiredreverberation effect output, the rheostat elements 3l connected to theplayback coil ll of all the playback heads are interconnected in seriestothe playback amplifier channel lead 46-A. Similarly, in order tosupply the required reverberation eifect to the loudspeaker R2, therheostat elements 3l connected to the coil l2 of all the playback heads,except the playback head PI, are connected to serially combine theiroutput and impress it on playback channel lead AG-B; and the rheostatelements 3l connected to the coll 13 of all playback heads, except theplayback heads PI, P2, P3, are connected to additively impress theiroutput on the playback channel lead 48-C leading to the playback amplierPB-3. Each playback amplifier PB-I to PIB- 3 may be provided with asuitable tone control arrangement TC and an attenuation controlarrangement AC, so as to permit amplification and adjustment of thecharacteristics of the amplified output impressed on the loudspeakers soas to produce the desired acoustic effect.

Instead of using an arrangement of the type described in Fig. 2 foroperating a plurality of diiierent reverberation channels for supplyingartificial acoustic treatment to a room, the different reverberationchannels may 'be used, for instance, for supplying one type ofreverberation characteristic to the voice of a singer, the other to themusic of the orchestra, another to the choir, in lieu of three separatereverberation channels, as described in Fig. 1.

Fig. 3 illustrates diagrammatically an arrangement using differentreverberation channels REA, REB, REC operating with a single endlessmagnetic tape l32 for supplying dilerent reverberation effects todifferent groups of loudspeakers Rl, R2, R3 used, for instance, forgiving to a room, such as shown in Fig. 2, the desired acoustictreatment, the arrangement of Fig. 3 operating in the same Way as thatin Fig. 2, except that each reverberation equipment REA, REB, REC isequipped with a recording head and a full complement of playback heads,such as described in connection with Fig, 1, for independently producingany desired reverberation effect or other time delay effects with asingle endless magnetic tape recording system.

When using arrangements, such as shown in Figs. 2 and 3, for supplyingartificial acoustic treatment to a room, the distances between theplayback heads are so arranged as to operate the different loudspeakersor sound reproducers with a time delay relatively to the sourcecorresponding to the time required for the sound to travel from thesource to the location of the loudspeakers. However, in many cases, forinstance, if special sound effects are desired, the sound delay suppliedby the magnetic recording arrangement may be made greater than theactual time required for the sound to travel from the source to theparticular part of the room. It is thus possible to give a roomartificial acoustical enlargement.

Although for ideal conditions, the room should be practically sounddead, it will not be necessary in practice. Thus, if the room has arelatively small reverberation time, the acoustic effects added by timedelay recording arrangements described in connection with Figs. 2 and 3may be only used to add .acoustic liveliness or other desired acousticcharacteristics to a room.

An endless magnetic tape recording device of the type designed inaccordance with the principles of the invention to operate as a timedelay medium, in the way described in connection with Figs. 1 to 3, isalso ideal for supervising the transmission of messages, for instance,broadcasts, to prevent dissemination of improper information, such as ofwar secrets during news broadcasts in wartime, An arrangement of suchtype exemplifying one form of the invention is shown in Fig. 4.

A source of signal energy, such as a microphone MC, delivers its outputto an amplifier IA, the output of which, instead of being impresseddirectly on a modulator and radio broadcast transmitter TR, is suppliedto a recordin-g head 4-R of an endless magnetic recording arrangementshown in the for-m of an endless magnetic tape 3| driven at uniformspeed in clockwise direction, for instance, in the Away described inconnection with Fig. 1.

'I'he signal recorded on each element of the moving endless tape remainsthereon until, after a full operating cycleof the endless tape, itreaches a playback head d-P, the output coil of which is connected tothe modulator and radio broadcast transmitter TR, so as to bring aboutthe broadcast of the message from the microphone in the same way as ii'it had been directly supplied to the radio broadcast transmitter. Anobliterating head 4 0, shown in the form of a permanent magnet, isplaced between the recording head 4-R and the adjacent main pickup headl-P, so that each element reaching the recording head is ready toreceive a new recording.

The equipment for supervising or censorng the transmitted message isshown as consisting of two censorng stations CA, CB, although morestations may be provided. Each censorng station is provided with a setof four push button switches K, LI, L2, L3, and with headphones 8|, asignaling lamp 82 and auxiliary switching elements for performing thevarious operations as explained hereinafter.

Upon actuating the push button LI of each station, the headphones 8| ofthe station are connected to an output lead 83 from the amplier iA, soas to enable the censor to listen to the transmitted message as it isbeing recorded on the endless magnetic tape 3| for subsequenttransmission, after the time delay determined by the recording length ofthe magnetic tape, for instance, one; minute, to the radio broadcasttransmitter TR. Upon releasing the push button Ll, and actuating thepush button L2, the censor at each station may connect his headphones tothe output lead of a playback amplifier AM-l upon which is impressed theoutput of a playback head located on an intermediate portion of the tapeso as to enable the censor, if,

AM-Z which amplifies the output of a third playback head 4-P3 placed adistance ahead of the main playback head 4-P, and as soon as he hearsthe initial phrase of the objectionable message, he has merely to pushbutton K which completes a circuit sending an obliterating current tothe obliterating head 4-03 placed ahead of the playback head 4-P,thereby cancelling the message recorded on the tape and preventing itstransmission when the elements of the tape on which it was recorded passthrough the main playback head 4-P.

In order to make such censorng system eiective, at least two censorngstations, such as-gindicated at CA, CB, must be provided so that whileone censor traces an objectionable message and attends to its beingcancelled, the other censor may attend to the censorng of the furtherpart of the lmatter that is being broadcasted. Under normal transmissionof a broadcast, only one censor has to watch the transmissionof thebroadcast.

In order to indicate to the censor that it is his duty to listen in, thesignaling lamp of the proper station, for instance, station A in thecase shown, is lighted by a circuit completed through the contact of astepping switch 88 having an operating coil B1, which, uponenergization, advances the contact of the stepping switch one step,thereby cutting out thel signal lamp of station A and lighting up thesignal lamp of station B, indicating to the censor oi the latter stationthat it is his duty to watch the transmitted message by closing his pushbutton switch Li.

With this arrangement, censor A, whose signaling lamp 82 is lighted andhas his push button Ll operated to listen to the transmitted message. onnoticing that the message contains objectionable matter, merely releasesthe push button Li and actuates the push button L2, which immediatelyconnects his headphones to the intermediate playback head 4P2 forenabling him to pick up the starting phrase of the objectionablemessage. On closing his push button switch L2, the censor at station Aalso completes a circuit including the normally closed contacts of thetwo auxiliary relays 88, 89 to the operating coil of the steppingswitch, thereby advancing it to the next step in which his own signallamp 82 is extinguished and the signal lamp 82 of the other censor islighted up, whereupon the censor B actuates his push button switchconnecting his own headphones to the amplifier IA for watching thetransmitted message.

In the meantime, the censor at station A, upon having picked up' thebeginning phase of the objectionable message, is releasing his pushbutton L2 and actuating his push button L3 so as to connect his headphone to the playback head 4-P3 and be ready to cancel the message assoon as he hears the initial phrase of the message in question,whereupon he actuates his cancelling button K for sending anobliterating current to the obliterating head 4-O3.

In order to prevent accidental obliteration of a message that is beingtransmitted, the obliterating push button switches K are normally lockedby a locking relay 9| which is actuated to release the obliteratingswitch only while the cancelling push' button L3 of one or the other ofthe two stations is actuated to cancel the message, thereby completing acircuit to the operating coil of the locking relay 9|. A foolproofcensoring system, which prevents interference with the transmittedmessage, but enables tracing of an objectionable part of the messagewhile its transmission is delayed, and its obliteration before it istransmitted, is thus made possible.

Obviously, a larger number of message tracing playback heads, such asl--P2 may be provided for enabling the censor to pick up dierent partsof the message which are stored in the endless tape, while they arebeing advanced from the recording head 4-R, during the time interval ofthe full endless tape recording cycle, to the point where they arepicked up by the playback head l-P for delivery to the radio broadcasttransmitter TR.

The endless magnetic recording devices of the type described in mycopending application Serial No. 197,851, led March 24, 1938, which willissue as Patent 2,224,854, and described therein in connection withFigs. 1, 5, 10 and 19, and which have proved successful in actualoperation, lend themselves for utilization as a time delay medium forsupplying artincial reverberation and for similar applicationsconstituting the subject matter of the present application. In Figs. 5and 6 is shown one form of arrangement for utilizing a magneticrecording device of the type described in connection with Figs. 8 to 10of my copending application Serial No. 197,851 as a time delay recordingequipment supplying artiiicial reverberation, in the way described inconnection with Fig. 1, for instance.

Inside a suitable casing, not shown, is mounted a rectangular framestructure 5-0 consisting of a front frame l-I and a similar rear frame 52 formed of metallic angle bars reinforced by corner braces and suitablyheld together by cross bars or struts. In the comers of the rectangularframe are mounted four cylindrical guide rollers 5-6 having bearingsjournaled on stationary shafts supported in the comer braces of theframe structure.

A long endless magnetic carrier in the form of a magnetic tape 5-13 iswound over the four guide rollers in the form of a helix, a suitablyguided cross loop completing the endless tape. The frame with the fourrollers forms thus a spool-like structure 0n which the endless tape maybe readily wound. One of the guide rollers is driven through a belt by amotor, not shown, suitably supported within the frame structure so as toimpart through the frictional driving engagement between themotor-driven roller with the portions of the tape helix engaging itssurface a uniform speed to the helix along the guide rollers. Suitablecomb bars maintain the helices in their proper position along therollers.

In order to make it possible to place a large number of magneticrecording and playback heads along a portion of the endless tape, thespool-like frame structure has mounted on one side thereof, on arms 5-2lsuitably secured to one side of the frame structure, an elongatedsupporting bar 5-25, shown as having a T- shaped cross section, and anelongated slot 5-28 is formed in the forwardly facing portion of the bar5-25 so as to enable adJustable mounting thereon of a series of magneticheads 5-21.

The magnetic heads are preferably of the type described in connectionwith Figs. 34 to 42 of my copending application Serial No. 340,030, ledJune 12. 1940, and as indicated in the drawing. each magnetic headconsists of two halves placed on opposite sides of the magnetic tapeS-II and held in position by a bracket 5 2! which is adjustably clampedto the front face of the supporting bar 5-25 by a bolt extending throughthe elongated slot 5-28 thereof. The location of the individual magneticheads and their relative spacing may be readily adjusted along thesupporting bar 5-25.

On the upper and lower ends of the supporting bar 5-25 are journaled twoaligning rollers l-JI for guiding one loop brought outwardly from theloop helix guided on the rollers 5-6 so as to pass through thesuccession of magnetic heads l-ZI mounted on the bar B-25, an auxiliarycrossover roller H2, suitably mounted on a bracket A extending from theupright mounting bar -2l,

leading the helix loop passing through the magnetic head from itsposition on the frame rollers 5-26 to its path of motion between thealigning Irollers B-Sl.

By using magneticheads of the type shown in Figs. 41 and 42 of the aboveapplication Serial No.

340,030. so that the coils in the pole pieces of vsuccessively arrayedheads placed in adjacent rows overlap each other, using one set of headsfor cooperating with one portion of the tape and the other set of headsfor cooperating with an adjacent portion of the tape, so as to 'utilizea single tape with two recording tracks, it is possible to playback arecorded signal with shorter time delay intervals between successiveplaybacks, such arrangement requiring, of course, a record ing head foreach playback track. The foregoing, of course, applies to an endlesstape driving arrangement, while the arrangement shown in Figs. 41 and 42is intended to illustrate an oscillatory tape drive. The foregoingprinciples are also ap plicable for an arrangement in which more thantwo'adjacent recording tracks are 'provided on a single endless tape.

For the sake of illustration, Fig, illustrates a series. of playbackheads PI, P2, etc. to PH, which are utilized for supplying artificialreverberation in the arrangement of Fig. 1, mounted successivelyadjacent to each other on the supporting bar 5-25 and enclosed in amagnetic shield 5-35, a recording head R and an obliterating head Obeing mounted ahead of the first playback head Pl outside the shieldenclosing the playback heads. An additional playback head is also shownmounted ahead of the obliterating head so as to permit utilization ofthe device shown as a censoring equipment. There is also secured to themounting bar 5-25 ahead of the magnetic recording and playback heads aswitch structure 5-5I which is utilized for automatically cutting outthe operation of the playback heads each time the soldering junctionpasses them, in the way explained hereinabove.

As shown in Fig. 6, any required number ci guide bars 5-25 with sets ofmagnetic heads mounted thereon in the way shown in Fig. 5, may besuitably mounted along one side of the spoollike guide roller structureS-l, 5-2 for guiding dierent loops of the endless tape helix 5-l3 overdifferent sets of magnetic heads, thus enabling the operation of adevice, such as shown in Figs. 5 and 6, with different reverberationchannels or in the way described in connection with the variousapplications described in connection with Figs. 1 to 4.

In order to support the individual guide bars 5--25 of the magneticheads in the different locations along one side of the frame structure,two angle bars 5-4|, for instance, may be detachably secured to the sidesurfaces of the front and rear frames 5-l, 5-2 of the spool-like rollerstructures, and the mounting arms 5 2! for carrying the guide bars 5-25are suitably secured, as by bolts, to the two cross arms 5-4I.

In Figs. 7 and 8 is shown the construction of the switch mechanism 5-5I,of the type shown at 5| in Figs. l to 4, for performing the controllingoperation which automatically cuts oil the output of the playback headswhen the soldering point passes through them, or for performing anyother desired signaling operation in conjunction with the operation ofan endless magnetic tape recording arrangement.

On forwardly projecting wall portions 5-52 of a supporting block 5-50,the back side of which is suitably clamped to the front face of theguide bar 5-25, are revolvably mounted two grooved guide rollers 5--53aligned so as to provide a guiding support for the tape 5-l3 as it movesupwardly towards the magnetic heads in the way shown in Fig. 5, twospring arms 5-54 CII serving to yieldably retain the tape 5-I3 withinthe groove of the rollers as it is being impelled at a uniform speedtoward the magnetic head. An additional biasing spring 5-55 urges thetape 5|3 to engage with its opposite edge the side walls of the tworollers 5-53 along which it is guided.

The control switch 5l for performing the control operations described inconnection with Fig. l. is shown in the form of 'a commerciallyavailable unit, such as sold under the name Micro switch, and designed,for instance, in the way described in Patent 1,780,758, to release thecontact from its open position shown to the closed position, if itsoperating arm 5-56 pivotally mounted on a bracket extending from theswitch casing releases the operating pin 5--51 of the switch to theoutward position under the biaslng action of the flexed spring contactof the switch 5i.

The free end of the operating arm 5-56 is provided with a bracket onwhich is pivotally mounted a grooved roller 5-58 engaging the facingedge of the tape so as to ride thereon as the tape is being impelled,and keep the switch operating pin 5--37 in its inward position in whichit holds the switch contact 5i open, and to release the switch 5| so asto close during a short instant while the depression 5-39 formed alongthe edge of the tape moves past the roller .tl-58.

Various other switching arrangements may be used to perform in a similaror analogous manner for automatically performing control operations inresponse to an automatic actuation by a signal element, such as adepression formed on the tape, so as to synchronize a. control operationwith a predetermined selected position of the endless tape as it isbeing continuously impelled.

In the guide and driving arrangement for an endless magnetic recordingtape described above, the magnetic tape is guided and driven at a uni-1form speed along a helix coiled around the outer surfaces of a pluralityof spaced guide rollers, one oi' the guide rollers having frictionaldriving engagement with loop portions of the helix being driven byexternal forces for impelling the tape helix past the magnetictransducing means, such as the recording and playback heads, forrecording and reproducing sound, at least one guide roller beingfrictionally driven by the moving tape helix and having sufficient massso as to exert on the tape helix inertia reaction forces which suppressfluttering of the tape helix and maintain it at substantially uniformspeed.

' In addition, the spacing of the guide rollers is designed in the waydescribed in my aforesaid application Serial No. 197,851, filed March24, 1938, which will issue as Patent 2,224,854, so that the spacingbetween the guide rrollers is suiliciently large and the effective guideroller surfaces engaged by the tape helix are sufficiently small topermit lateral slippage of the tape portions of the helix over theroller surfaces while they are impelled along the path of the helix.

In addition, the effective length of the helix portions frictionallyengaged with the guide roller surfaces is made sufilciently small andthe effective length of the free helix portions extending between thespaced guide rollers is made sufficiently large as to permit limitedslippage of the tape helix portions over the guide roller rollersurfaces to assure uniform speed along its helical path.

Furthermore, the effective length of the helix portions frictionaliyengaged with the guide roller surfaces are so proportioned in relationto the effective length of the free helix portions, and the inertia ofthe guide roller masses frictionally driven by the tape helix aredesigned to be sufciently large as to permit limited slippage of thetape portions over the guide roller surfaces while being kept insufficiently effective frictional engagement therewith for maintaininguniform speed of the tape along the helix.

Furthermore, the frame for supporting the guide rollers is arranged toform a spool-like structure permitting winding of the endless tape helixaround the outer roller surfaces when it is being joined into an endlesstape, and the interior space of the Spool juncture may be readilydesigned as a support for the driving motor mechanism and the amplifiermeans associated with the operation of the endless magnetic taperecording system.

In Figs. 9 to 14 is shown an endless magnetic tape recording devicedesigned in accordance with the principles disclosed in connection withFigs, l, 5, 13 and 27 to 29 of my copending application Serial No.197,851, but embodying various desirable improvements thereover,exemplifying various other features of the invention. Inside a casing,not shown, for the sake of clarity, is mounted a rectangular framestructure consisting of a front frame 9 5, and a rear frame 9 2 ofmetallic angle bars reinforced at the corners by brace plates 9 3, 9 4,9 5, 9 6 and held together by cross bars or plates 9 1, 9 8, 9 9, thelatter being clamped to the intermediate angle bars 9 Il secured betweenthe side members of the front frame 9 l and the rear frame 9 2, in theway shown in Figs. 9 and 10.

In the form shown, two cylindrical guide rollers 9 2I are revolvablymounted on shafts 9 22, the ends of which are supported by the cornerbraces 9 3, 9 4 shown in the lower part of the frame structure, and twopairs of additional rollers 9 23, 9 2# are similarly revovably mountedon shafts suitably held by the corner brace plates 9 5, 9 6 on theopposite part of the frame structure. A long endless magnetic tape 9 3iis would in the form of a helix, first as an inner helix layer, forinstance, by starting on the left sides of the rollers, as shown in Fig.9, and winding it from the left to the right around the two pairs ofrollers 9 2I, 9 23, and then back from the right side of the rollers, asshown in Fifz. 9, in the same direction, but on the two pairs of rollers9 2I and 9 24, back to the left end of the rollers, shown in Fig. 9, toa point where the end of the outer helix joins the starting point of theinner helix, so as to form a continuous endless tape. Two layers of tapeare thus would on the lo'wer pair of rollers 9 2l, and one layer of tapeis would on each of the other two pair of rollers,9 23, 9 24.

In order to guide the successive loops of the endless tape 9 3I alongthe helices so formed, comb-like bars 9 32 are mounted in front of a fewof the guide rollers, for instance, in front of the guide roller 9 2Iheld by the brace plate 9 3, and the guide roller 9 23 held by the braceplate 9 5 in a diametrically opposite corner of the frame structure, sothat if the tape is irnpelled in clockwise direction, each loop of thetwo tape helices will be retained in its proper operating positionrelatively to the other loops forming part of the endless tape.

An electric driving motor 9-34 has its vibration absorbing bracketstructure 9 35 mounted on the cross plate 9 1 of the spool-like framestructure so formed, and a driving pulley mounted on one end of themotor shaft drives through a rope belt 9 36 one of the guide rollers 923 so as to impart to the tape a uniform motion, in clockwise direction,for instance, as indicated by the arrow.

In the design of such endless magnetic recording tape guide and drivingarrangement, the following considerations are important: If the drivin'garrangement is to be used for synthetic reverberation, it is of utmostimportance to eliminate even traces of flutter due to irregularities ofthe motor speed or any other causes, because if even slight changes ofthe momentary tape speed occur, the signal will be picked up by thedifferent playback heads with a varying phase shift givinguncontrollable differences in pitch. Since the signal reproduced by thereverberation playback heads is combined with the original signal, suchdifferenecs in pitch would bring about a modulation effect causingundesirable disturbance of the artificial reverberation supplied by suchrecording arrangement.

By using an endless rope belt 9 36, of the type employed in dentaldrives, and winding the belt in the form of a plurality of loops so asto be guided in suitable grooves formed on the motor pulley andcorresponding grooves formed in the periphery of the motor driven guideroller 9 23, such belt may be operated with sufficiently small tensionper belt loop as to take up in the belt any fluctuations of the motorspeed without transmitting them to the motor driven roller 9 23. Suchdriving arrangement is also of great advantage because it reduces thebelt tension and wear and also reduces the friction losses. As a result,a smaller motor may be used and the remanent mechanical noisesinherently present in any mechanical drive is brought down to a very lowlevel.

As shown in Figs. 10 and 14, the correct tension of the -belt 9 36 ismaintained by a cross-over pulley 9 31 journaled on the end of an arm 938 which is pivotally mounted on a bracket extending from the motor, asuitable biasing spring 9 36 serving to supply the pressure with whichthe cross-over roller 9 31 maintains the crossover loop of the endlessbelt 9 36 at the desired tension. The end grooves of the motor pulley ofthe motor driven guide roller 9 23 are sufciently wider than the groovesover which the other loops of the endless belt 9 3; are wound so as toenable the cross-over loop to be guided from the point where it leavesthe motor pulley to the point where it enters the starting point of thebelt helix on the motor driven guide roller 9 23. One of the loops ofthe rope belt 9 36, which is not used for transmitting the driving forceto the guide roller 9 23, is utilized as the cross-over loop.

In order to secure the full effectiveness of such drive in suppressingeven traces of flutter, the magnetic heads are arranged to engage withtheir pole pieces one or more loops of the inner tape helix layer guidedover the guide rollers 9 23 so as to impart to the inner tape helix thetension required for maintaining its loop portions that pass over theguide rollers 9 2I, 9 23 int 1good frictional driving engagement therewiIn order to assure long foolproof operation without requiring servicingof the moving parts, the guide rollers are revolvably mounted on theirshafts 9-22 by ball bearings 9-42 in the way shown in Fig. 12. Comparedto sleeve bearings, ball bearings have the great advantage that theyoperate with less noise than sleeve bearings, a factor which is veryimportant in such magnetic tape drives, for instance, when used as areverberation medium in the studio where the sound to which thereverberation is applied originates. The ball bearings are interposedbetween the shaft and the rollers so that the shaft remains stationarywhile the roller revolves in the way shown in Fig. 12.

The ends of the shaft 9-22 are supported by a resilient member, forinstance, Neoprene, in the way shown in Fig. 12, in order to assure thatany remanent mechanical vibration imparted to the shaft is nottransmitted to the frame structure 9-|, 9-2 having a large soundradiating u are ,from which noise should be kept away. In ord r toreduce the noise, the surface of the guide rollers 9-2I, 9-23, 9--24 ismade of non-metallic sound-absorbing material. Since such nonmetallicsoundabsorbing material is usually an electrical insulator, it willaccumulate in the course of operation electric charges due to frictionwith the tape, and such charges,r if accumulated until they reach apotential sufficient to spark over to the frame structure, will induceelectric noise effects in the playback circuits associated with therecording and playback equipment. This disturbing effect is eliminatedby providing, as shown in Fig. l2, a sliding metallic contact member9-45, supported by the metallic frame structure and pressing against themetallic portion associated with the shaft, such as the inner ball raceof the ball bearings 9-42.

It has been found that the guide bars, such as shown at 9--32 forguiding the loops of the helices in th'eir positions along the rollers,may cause a disturbing high pitched noise caused by the mechanicalvibrations of the comb teeth, as the tape loops which they keep apartmove past them.

Fig. ll illustrates a comb guide bar arrangement 9--3I for the tapehelices which' was found to be highly efficient in properly guiding theloops of the tape helices, without causing any noise. As shown, a bar9--3I of suitable nonmagnetic material, such as brass, for instance,holds in suitable grooves or holes formed therein tooth members 9-5l inthe form of pins or tooth-shaped sheet metal portions projecting betweenand separating adjacent loops of the helix, two strips of felt 9--52,9--53 being held in position on the teeth so as to confine the loops ofthe tape and the projecting teeth portions between them, and suppressany vibrations of the same.

The two felt strips 9-52, 9-53 are held suitably clamped, for instance,by an overlying bar 9-54, likewise'of non-magnetic material, which issuitably attached to the comb bar 9-3l, for instance, by screws, asshown in Fig. 11. This arrangement of .the guide bars not onlysuppresses vibrations of the tape and of the guide bar teeth, but alsoautomatically cleans the tape.

In addition, it distributes the tension of the tape uniformly over itsindividual loops.

Obviously, an endless magnetic tape guide and drive arrangement of thetype shown in Fig. l

`may be, of course, made to operate with a larger or smaller number ofsuch rollers. For instance, the endless tape may be wound in a similarway in the form of two helix layers over only one roller 9-2I and oneroller 9--23 and one roller 9--24, the three rollers being supported ontwo frame` members, such' as two side members of the frame structure 94, 9-2 interconnected by a cross plate 9 8, in the way shown. Suchendless magnetic tape driving arrangement may be used in the variousapplications outlined in my vprior application Serial No. 197,851, filedMarch 24, 1938, which will issue as Patent 2,224,854, and it is desiredthat the specification of said applications, together with its drawings,should be considered as a part ofthe present application.

The endless tape guide and drive arrangement shown in Figs. 9 to 14 isparticularly suited for use as a time delay medium for supplyingartificial reverberation or other acoustic effects requiring theoperation of the magnetic tape with several sets of magnetic recordingand playback heads arranged in different magnetic tape portions.

As shown in Figs. 9, 10 and i3, the frame structure 9|, 9--2 also servesas a support for three additional rollers 9--6I, 9-62 mounted in a waysimilar to the other guide rollers in three corners of the framestructure. Any number of loops of a continuous endless tape guided anddriven on the inner rollers 9-2i, 9-23, 9-24 may be led from theirposition on the inner rol 1ers so as to move over the outer rollers 9SI,9-62,ifor instance, the lst, the 11th, the 31st loop of the inner helixor similar loops of the outer helix may be guided out of their innerposi tion to the position along the outer rollers S-SI, 9-62.

In the arrangement shown in Figs. 9 and l0, two loops 9-63A, 9-63B areshown guided from their position along the inner helix moving over thetwo pairs of rollers 9-2I, 9-23 so as to move over the outer rollersS-l, 9-62 and return to their positions in the inner helix, suitablegaps being provided when winding the loops of the outer helix around therollers 9--24 for permitting the loops 9-63A, 9-63B to be led from theirposition on the inner helix to their position on the outer rollers9--6|, 9-62.

As shown in Figs. 9 and l0, angle metal cross bars 9-'ll are suitablysecured to the brace plates of the two frames 9-i, 9-2 so as to providemounting supports for guide bars 9-l2, 9-13 on which any desired arrayof magnetic heads may be mounted for cooperation with the tape loops9-63A, 9-63B passing thereover.

As shown in Figs. 9 and l0, channel members are used for the guide bars9--l2, 9-13 and an elongated slit 9-14 is provided in each guide barpermitting adjustable mounting thereon of the bracketsr 5-28 of themagnetic heads, as by screws 9-'I5, and nuts, in the way shown. Amagnetic shield 5-35 for enclosing the magnetic heads, as in thearrangement of Fig. 5, is likewise arranged for detachable mounting onthe guide bars 9`H, 9-13.

To secure maximum effectiveness in suppressing flutter, in a tapedriving arrangement of the type shown in Figs. 9 and 10, the magneticheads should be placed on a tape loop portion which has to traverse atleast one of the tape driven guide rollers, such as 9-2|, 9-24 before itreaches the motor driven roller. Thus, in the arrangement shown i-nFigs. 1 and 2, the portion of each tape portion 9-3I leaving themagnetic heads located on an outer loop guided over the outer rollersS-SL 9-82, has to pass over at least four tape driven rollers 9-2i, 8-24along the outer helix and the two tape driven rollers 9-'2l along theinner helix before coming into driving engagement with the surface ofthe motor driven roller 9-23. One of the driving belt loops which doesnot transmit the driving iorce is used as a cross-over loop, and thebelt idler roller 9-31 urges this cross-over loop in outward directionso as to enable the cross-over roller to guide the cross-over loop fromthe point of the motor pulley where it leaves the end of the belt helixto the point on the roller where the belt helix starts, withoutinterfering with the other loops of the belt 9-I8.

The use of the roller arrangement of the type shown in Figs. 9 and 10makes it possible to separate the convolutions of one or more outer tapehelix layers guided over the outer guide rollersV 9--24 from the innertape helix guided over the rollers 9-23 over the major portion of itslength without requiring much more room for guiding' a very long endlesstape, since only a relatively short portion of the loops of the innertape helix has superimposed thereon portions of the tape helix woundaround it. This is not objectionable because the overall length of thetape is made sufficiently long and so related to the frictionalengagement o! the tape loops of the inner tape helix llayer with thedriving rollers as to leave in the outer tape helix sulcient slack toassure that no portion of the outer tape helix is under excessivetension and moves loosely over the inner tape helix.

A larger number of additional helix layers may be wound in pairs overthe outer helix layer engaging the rollers 3-24, one layer of each pairof outer helices being wound around the spoollike structure formed bythe rollers 9-24, 9-2I, in the same direction as the inner helix layershown in Fig. 13, and then back in the direction or the outer helixlayer, the end of the outermost layer of the last pair being Joined tothe end of the inner helix layer wound around the rollers 9;-23, 9--2 I.By employing the construction of the type shown in the drawing, a greatlength of endless tape can be guided and driven with a small spool-liketape guiding and driving structure. The size of the guide rollers sdetermined by the tape thickness. It is important that the curvature ofthe rollers shall be large enough to assure that the bending to whichthe tape is subjected in passing thereover does not impose on itstresses that eilect its magnetic properties. Thus, a magnetic tape oftungsten steel having a thickness of three thousandths of tape rollershaft 9-22 on which the tape roller the guide roller in the adjustedposition, in the way shown in detail in Fig. l2.

It two layers of magnetic recording tape are permitted to remain inengagement with each other and are subjected to excessive pressure, afraction of the magnetic energy of a recorded signal stored in one tapewill be impressed on the adjacent tape layer resulting in undesirableecho eiects when reproducing the, original record. By adjusting the tapein the way described above, such undesirable eilects may be eliminated.

As pointed out, hereinafter, when a magnetic tape machine of the typedescribed herein is used as a time delay device, for instance, forproducing reverberation effects, the undesirable echo eiects describedabove are of no significance because only the portion of the tape whichis engaged by the magnetic heads mounted immediately followingtherecording head is utilized for performing the operation.

In special applications where it is essential to suppress even theslightest traces oi undesirable echo effects, the outer tape layersextending over the rollers -9--2I on which the inner as well as theouter helix layers are guided, may be separated by the teeth of theguide combs over adjacent guide roller surface portions so that one tapelayer does not engage another. However, if provisions are made to assurethat the loops of the innermost tape helix wound around the innerrollers '9-3l are held suillciently tensioned to maintain the drivingengagement of its loops with the motor driven rollers, sufticlent slackmay be provided as to assure that at all times the outer tape helixlayer is suiiiciently loose to positively eliminate any traces ofundesirable echo effects. With such arrangement, any required number ofouter helix layers may be wound and operated over the first outer helixprovided the outer tapes have suiilcient slack as to enable them toslide freely along their path without being subjected to excessivepressure.

.According to the invention, the pole pieces of the magnetic heads,which are used for recording and reproducing the signal, are utilized toexert sufiicient friction on the inner tape helix as to maintain it inthe required operating tension. The magnetic heads lmay be supported inthe space of the inner helix as described in my prior applications.However, in many cases, such as in time delay machines used forreverberation, where a large number of magnetic heads have to beemployed, it is desirable to mount the heads so that they are accessibleon the exterior of the supporting structure. According to the invention,one or more loops of the inner tape helix layer are brought out andguided in the space beyond the outer tape helix so as to make itpossible to guide such outwardly brought tape helix through magneticheads mounted in such space.

As shown in the arrangement of Figs. 9, 10 and 13, a suitable number ofloops of the inner helix are brought out through gaps lett in the outerhelix layer and guided over the outer guide rollers so as to guide twosides of each loop along the periphery of the frame structure through aplurality of magnetic heads mounted along them, and thus maintain theinner helix at the proper tension, while leaving the outer helix layerssufilciently loose as to permit them to slide past each other.

Since a large number of magnetic heads placed along these helixesautomatically subject the inner helix to a substantial friction, theloops

